1. Field of the Invention
This invention pertains to an improved cyclic steam stimulation method for the recovery of viscous oil from subterranean, viscous oil-containing formations.
2. Background of the Invention
Steam has been used in many different methods for the recovery of oil from subterranean, viscous oil-containing formations. The two most basic processes using steam for the recovery of oil includes a "steam drive" process and "huff and puff" steam process. Steam drive involves injecting steam through an injection well into a formation. Upon entering the formation, the heat transferred to the formation by the steam lowers the viscosity of the formation oil, thereby improving its mobility. In addition, the continued injection of the steam provides the drive to displace the oil toward a production well from which it is produced. Huff and puff involves injecting steam into a formation through an injection well, stopping the injection of steam, permitting the formation to soak and then back producing oil through the original injection well.
Heavy or viscous oils go through significant visbreaking upon heating at temperatures normally above 400.degree. F. Such reduction in viscosity significantly enhances recovery during a thermal operation and improves the quality of heavy oils.
In copending application Ser. No. 331,424, now abandoned, filed Dec. 16, 1981 to W. R. Shu et al, there is disclosed a method to induce maximum in-situ visbreaking of heavy oil by injecting high temperature steam into the formation via a well having a selected completion interval and producing oil from the same well used for the injection of steam.
We have found that the cyclic steam stimulation method can be improved to induce maximum visbreaking of the heavy oil in place if the formation contains no underlying water zone, the well completion interval is at least 80% of the vertical thickness of the formation, and the steam is injected as rapidly as possible.